Lymphocyte transforming factor and other soluble factors released from stimulated human lymphocytes will be studied for their role in the development and expression of B and T lymphocyte-mediated immunity. The proliferative response, the generation of cytotoxic lymphocytes in the mixed leukocyte culture and against tumor cells, and the development of antibody-forming cells all will be examined. Attempts will be made to purify and characterize lymphocyte transforming factor.